1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer for transferring onto a recording material a toner image formed on an image bearing member using, for instance, an electrophotographic process and subsequently fixing the toner image to form a permanent image on the recording material.
2. Related Background Art
Up to now, as color image forming apparatuses capable of outputting a full color image, apparatuses having the following structure have been put in practical use. That is, at a first transferring area formed in an abutting part between an image bearing member surface and an intermediate transferring body surface, a first transferring bias is applied to a first transferring member disposed on a rear side of the intermediate transferring body to temporarily transfer onto the intermediate transferring body surface a toner image on the image bearing member surface (hereinafter, referred to as “primary transfer”). After that, a transferring material passes through a second transferring area formed at a contact part between the intermediate transferring body and a second transferring member to apply a second transferring bias thereto, so that the toner image on the intermediate transferring body surface is transferred again onto the transferring material (hereinafter, referred to as “secondary transfer”).
FIG. 9 shows an example of the color image forming apparatus having the above structure. Now, referring to FIG. 9, a description will be given of an operation of the image forming apparatus having the above structure.
In the image forming apparatus of this example, image exposure using a laser beam L is applied from an exposure apparatus 103 through a reflection mirror 104 onto a rotation drum type electrophotographic photosensitive member (hereinafter, referred to as “photosensitive drum”) 101 as the image bearing member rotating in a direction of an arrow R1, which is uniformly charged by a charger 102. Then, latent images corresponding to target color images are respectively formed on an exposure area A.
Next, the latent images are developed by a developing devices 105 (yellow developing device 105Y, magenta developing device 105M, cyan developing device 105C, and black developing device 105Bk) to thereby form an yellow toner image, a magenta toner image, a cyan toner image, and a black toner image on the photosensitive drum 101, respectively. The yellow toner image, the magenta toner image, the cyan toner image, and the black toner image are superposed in order on a surface of an intermediate transferring belt 106 at a primary transferring nip part B as a primary transferring area between a primary transferring roller 107 and the photosensitive drum 101 and primarily transferred thereonto. This primary transfer was carried out while the intermediate transferring belt 106 makes four rotations in a direction of an arrow R2. The toner images of full color thus superposed on the intermediate transferring belt 106 are collectively secondarily transferred as the full color toner image corresponding to a target color image onto a transferring material P fed to a secondary transferring nip part C as a secondary transferring area between a secondary transferring roller 108 and a secondary-transferring opposing roller 106b. The transferring material P, after undergoing the secondary transfer, is transported to a fixing device 115 where the toners of four colors are melted for color mixture by applying a pressure and heat thereto and fixed onto the transferring material P. Thus, a full-color final image is formed on the transferring material P.
After the above process is completed, a secondary transfer residual toner on the intermediate transferring belt 106 is removed by an intermediate transferring belt cleaner 109. In addition, a primary transfer residual toner on the photosensitive drum 101 is collected by a cleaner 110 and used for a subsequent cycle. Note that in such an image forming apparatus, a laser beam scanning direction is called a main scanning direction (direction perpendicular to a direction in which the photosensitive drum moves), whereas the directions of the arrows R1 and R2 in which the photosensitive drum 101 and the intermediate transferring belt 106 move, respectively are each called a sub-scanning direction. For the purpose of further improving an image quality of a final image obtained by the above image forming apparatus, however, the inventors of the present invention have made various studies on the image forming apparatus of such a type and found that, upon primarily transferring the toner image formed on the photosensitive drum 101 surface onto the intermediate transferring belt 106 surface, an abrupt rotation variation of the photosensitive drum 101 may occur, which causes an exposure unevenness of the laser exposure L. This leads subsequently to an image streak occurring on the toner image formed on the photosensitive drum 101 surface.
To cope with such a problem, dot-shaped toner images are additionally formed on the photosensitive drum 101 in addition to the toner image of an image pattern that a user demands, so that the photosensitive drum 101 surface and the intermediate transferring belt 106 surface at the intermediate transferring nip part B surface are made smooth to each other to thereby avoid the exposure unevenness resulting from the rotation variation, which is presumed to achieve an effect to a certain degree.
For example, in an image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H11-52758, minute dot toner images are formed on the photosensitive drum through a uniform dispersion to prevent a color drift from occurring on the toner image primarily transferred onto the intermediate transferring belt. By using the dot toner image, the same protection effect may be supposedly exerted on the image streak resulting from the exposure unevenness as well.
However, there remains to be solved a problem that a sufficient effect cannot be obtained in some cases depending on an arrangement of the additionally formed dot toner images.
For example, the dot toner images disclosed in Japanese Patent Application Laid-Open No. H11-52758 are formed at such a low density that about 3 to 10 dots are formed per unit area of 1 cm2. More specifically, according to Japanese Patent Application Laid-Open No. H11-52758, it is unsure whether or not the dot toner image is always within the primary transferring nip part. In particular, if the dot toner images are arranged in the sub-scanning direction at a larger distance, it is conceivable that no dot toner image is intervened in the primary transferring nip part at a certain timing during printing. Under such situations, the image streak cannot be stably prevented from occurring in every image pattern that the user demands.